in reply to Macro in perl code?

Use a subroutine with an empty prototype which return a constant (that's what the constant module does): 
perl -MO=Deparse,-x,-p -e 'sub verbose() { 0 } print "guggug!\n" if ve
+rbose;'
sub verbose () { 0 }
'???';
-e syntax OK

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perl -MO=Deparse,-x,-p -e 'sub verbose() { 0 } if(verbose){ print "gug
+gug!\n"};'
sub verbose () { 0 }
'???';
-e syntax OK

[download]

The '???'; statements stand for bits of code that have been optimized away; those will have no impact on performance. If the compiler sees such a constant returning sub it can perform optimizations, eliminating code blocks whose execution is bound to the value of that constant.

Besides, before shoehorning a perl script through m4, I'd rather look - though discouraged - at perl's -P switch. See perlrun.

--shmem 

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

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Re^2: Macro in perl code?
by pileofrogs (Priest) on Oct 16, 2007 at 20:32 UTC

    Sorry, I don't get it at all. Could you point me to some docs that explain this? I think I need to get more background..

[reply]
      Constant functions (with the empty prototype) are discussed in perlsub.
      Constant Functions Functions with a prototype of "()" are potential candidates for inlining. If the result after optimization and constant folding is either a constant or a lexically-scoped scalar which has no other references, then it will be used in place of function calls made without "&". Calls made using "&" are never inlined. (See con- stant.pm for an easy way to declare most constants.)

      See e.g. The Optimizer and Preprocessor Pranks for more details.

      --shmem 

      _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
[reply]

        You Rule!

[reply]
        Sorry for asking again, but I'm still unsure about how to use this. Could you explain/exemplify a bit more ?

        Would I have to have say 
        ...
sub X() { 0 } print "$message1 \n" if( runtime cond);
...
sub X() { 0 } print "$message2 \n" if( runtime cond);
...

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        ?!

        Why would that be better than 
        ...
print "$message1 \n" if( runtime cond);
...
print "$message2 \n" if( runtime cond);
...

        [download]
        I'm confused. Would be nice if you can shed a bit more light on it - thanks!
[reply]
[d/l]
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